Electron energy sensitive phosphors for multi-color cathode ray tubes

ABSTRACT

A method is disclosed for making multi-component phosphor structures whose emission color depends on the energy of the electrons used to excite them. The method comprises the mixing of two or more component phosphors, each component phosphor having particles with non-luminescent interior core regions and nonluminescent exterior surface regions separated by a luminescent region, the region dimensions and emission color of each component being distinct. These structures are formed by diffusing activating and/or coactivating impurities partially into the particles, or by growth of activated material on the outside of non-luminescent material, and by the subsequent diffusion of luminescence killing impurities partially into the luminescent region.

United States Patent Kingsley et al.

[451 May 23, 1972 154] ELECTRON ENERGYSENSITIVE PHOSPHORS FORMULTI-COLOR CATHODE RAY TUBES [72] Inventors: Jack D. Kingsley; JeromeS. Prener, both of Schenectady, NY.

117/33.5 E, 117/33.5 C, 117/33.5 CP, 117/33.5 CM, 117/71 R, 252/30l.6 S,313/92 PH [51] Int. Cl. ..C09k l/12, 844d 5/00 [58] FieldofSearch..117/100B,71 R, 27,33.5R, 117/33.5 E, 33.5 C, 33.5 CF, 33.5 CM;252/301.65; 313/92 PH [56] References Cited UNITED STATES PATENTS2,177,735 10/1939 Meyer ..252/30l.6 X 2,137,118 11/1938 Schleede.....252/301.6 X 2,289,978 7/1942 Malten ....252/30l.6 X 2,590,018 3/1952Koller et a1. ..252/30l.6 X 2,567,714 9/1951 Kaplan ..252/30l.6 X2,749,252 6/1956 Groner.... ....252/301.6 X 2,901,651 8/1959 Destrian......252/301.6 X 2,908,588 10/1959 Harper ..252/30L6 X 2,958,002 10/1960Cusano ..252/301.6 X 3,253,146 5/1966 de Vries... ..252/301 6 X3:275,466 9/1966 Ke1l....., 252/301.6 X 3,294,569 12/1966 Messineo etal. ..252/30l.6 3,330,981 7/1967 Aia 252/301.6 X

3,408,223 9/1968 Shortes ..252/301.6 X

3,449,148 6/1969 Shortes ..252/301.6 X 3,522,463 8/1970 Bishop.......252/301.6 X 3,523,905 8/1970 Carvell ..252/301.6 X

Primary ExaminerWilliarn D. Martin Assistant ExaminerMathew R. P.Perrone, ,1 r. Attomey-John F. Ahern, Paul A. Frank, Jerome C.Squillaro, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman[57] ABSTRACT A method is disclosed for making multi-component phosphorstructures whose emission color depends on the energy of the electronsused to excite them. The method comprises the mixing of two or morecomponent phosphors, each component phosphor having particles withnon-luminescent interior core regions and non-luminescent exteriorsurface regions separated by a luminescent region, the region dimensionsand emission color of each component being distinct. These structuresare formed by diffusing activating and/or coactivating impuritiespartially into the particles, or by growth of activated material on theoutside of non-luminescent material, and by the subsequent diffusion ofluminescence killing impurities partially into the luminescent region.

5 Claims, 6 Drawing Figures ACCELERATING VOLTAGE AT MAXIMUMEFFICIENCY-(KV) DEAD VOLTAGE, v

PATENTEnmAm I972 2 HOUR FIR/N6 TIME /0 M/NU TE FIR/N6 T/ME l l l l I l J800 900 IOOOIIOO I200 TEMPERATURE,C

FIG. 6

/2 HOUR F/R/NG TIME IO M/NUTE F/Pl/VG TIME O .I J L L 500 600 700 800900 FIRING TEMP (C) RELATIVE EFFICIENCY RELATIVE EFFICIENCY ACCELERATINGPOTENTIAL FIG. 3

ACCELERATING POTENTIAL N VE N TORS.

JACK D. K/NGSLEY, JEROME 5. PRE NE ELECTRON ENERGY SENSITIVE PHOSPHORSFOR MU LTI-COLOR CA'I'I-IODE RAY TUBES This invention relates toluminescent materials and more particularly, to methods for preparingphosphor particles containing both luminescent and non-luminescentregions arranged such that the efficiency with which the particlesgenerate light when they are excited by a beam of electrons depends onthe energy of those electrons in a controllable manner.

In nearly all present day color television tubes, the emission color iscontrolled by directing the exciting electrons to one of threephysically separated phosphors having a distinct emission color. Becausethe physical dimensions of the regions having distinct emission colorsmust be very small in order to provide a picture which is acceptable forviewing, complex tube structures are required, such as the conventionalaperture mask tube which has three electron guns and a metal platehaving several hundred thousand microscopic holes therein. Because ofthis complexity, conventional cathode ray tubes are much more difficultand expensive to manufacture than tubes capable of emitting one color,or white, radiation which consist of only one electron gun and a simplehomogeneous phosphor screen.

An alternate approach to controlling the emission color of a cathode raytube is that of the penetron principle such as those described by Kolleret al. in U.S. Pat. No. 2,590,018, Cusano et al. in U.S, Pat. No.2,958,002, and Kell in U.S. Pat. No. 3,275,466. In the classicembodiment of this principle, the phosphor screen consists of two ormore uniformly thin phosphor layers, each having a distinct emissioncolor. When such a tube is operated with a sufiiciently smallaccelerating potential, the electrons have an energy sufficient topenetrate only that phosphor layer closest to the electron gun, so thatthe screen emits the color characteristic of that phosphor layer. Athigher accelerating potentials, the average depth to which electronspenetrate into the layers, the range, is greater. Consequently, thosephosphor layers further from the electron gun are excited more stronglyas the electron energy is increased and the emission color changes.Because the penetration depth of the electrons is not precisely defined,but rather excitation occurs from the surface inward, to a maximumpenetration distance, the different phosphor layers cannot be excitedwith complete selectivity. The selectivity can be improved through theincorporation of transparent, non-luminescent inert layers, such asthose disclosed by Messineo et al. in U.S. Pat. No. 3,294,569.

In order to maximize the selectivity and color uniformity over thescreen, the phosphor layers in a penetron type tube must be thin andextremely uniform in thickness. Such layers are difficult and expensiveto make and tend to be optically inefficient.

To avoid the formation of the thin layers, Messineo et al. also proposesmaking multiple layered phosphor particles with each layer composed of adifferent phosphor material capable of emitting a different color.

A simpler and more practical approach is disclosed by Shortes in U.S.Pat. No. 3,408,223, wherein the phosphor particles are coated with anon-lumine'scentmaterial such as silicon dioxide or tin oxide in orderto reduce the response of the phosphor to low energy electrons. Whenthese coated phosphor particles are mixed with other phosphor particleshaving an inert coating of a different thickness and a differentemission color, one has a composite phosphor whose emission colordepends on the energy of the exciting electrons.

In such a composite phosphor having two components, both components willemit radiation when excited with sufficiently high energy. In order thatthe emission color of the component having the thicker inert coating isdominant at the higher operating potential, it is necessary to reducethe fraction of the total phosphor volume which is occupied by thecomponent having the thinner inert coating. As a consequence, the screenbrightness at the lower operating potential is necessarily reduced.

This difficulty can be avoided in accord with the teachings of theinstant invention by forming phosphor particles for one or more of thecomponents with efficiencies for the generation of light which decreasesabove some electron energy as the electron energizing level increases.This can be achieved by having particles whose central regions, orcores, are non-luminescent.

It is therefore the object of this invention to provide a new andimproved method for making multi-component electron energy sensitivephosphors in which each component phosphor has a distinct characteristicresponse to electron energy and a distinct emission color such that theemission color of the composite phosphor can be controlled by controlling the energy of the electrons used to excite the phosphor.

A further object of the invention is to provide a novel method formaking phosphor particles having a non-luminescent interior region, orcore, surrounded by a luminescent region such that the efficiency of theresponse of the phosphor decreases above some electron energy.

Another object of this invention is to provide a method for reducing oreliminating the response of the phosphor particles to electrons havingenergies less than some certain predetermined value by means of theformation of a non-luminescent region near the surface of the phosphorparticles.

Still another object of the invention is to provide a method of makingphosphor particles each having a non-luminescent core region and anon-luminescent surface region separated by a luminescent region, withall three regions having completely arbitrary and controllabledimensions, such that the electron beam energy at which the response isa maximum, can be predetermined.

Yet another object of this invention is to provide a method for making asimple homogeneous phosphor mixture which, when applied to the faceplate of a cathode ray tube by conventional techniques and excited byelectrons having selectively variable energies, is capable of displayinginformation or television pictures in a variety of colors.

Briefly, the present invention achieves these and other objects by meansof a novel process in which suitable non-luminescent phosphor particlesare first prepared, either by the inward solid state difiusion of thedesired activating and coactivating impurities to the desired depth,leaving the region of the phosphor particles farthest from the surfacenon-luminescent, while the region occupied by activating andcoactivating impurities becomes luminescent. Alternatively thisconfiguration can be achieved by the growth of a luminescent regionaround a non-luminescent particle. A non-luminescent outer surfaceregion can be made by the subsequent diffusion of a killer impuritywhich suppresses the luminescence of the region of the particle itoccupies. Electrons having low energies are stopped entirely in thenon-luminescent surface region while electrons having very high energiestend to penetrate into the non-luminescent central region, thus tendingto lose their energy without producing appreciable luminescence. Thoseelectrons having an initial energy such that they tend to lose thegreatest fraction of their energy in the luminescent region produceluminescence with greatest efficiency.

The size or thickness of the non-luminescent and luminescent regions canbe determined in a controlled manner through the selection of phosphorparticles of the desired size, by controlling the parameters of thediffusion processes or by controlling the parameters of the layer growthprocess. In this way, the threshold potential and the acceleratingpotential at which the phosphor efficiency achieves its maximum valuecan be controlled. Thus, either the non-luminescent core or thenon-luminescent outer layer may have dimensions equal to any fraction ofthe final phosphor particle radius ranging from zero to 1.

In accord with one embodiment of the invention, a composite phosphor isconstructed by means of simple mechanical mixing of two or morephosphors having distinct emission colors and efficiencycharacteristics. For example, a cathode ray tube having a two-componentphosphor constructed in accord with the instant invention when excitedat a low operating voltage, V,, emits color from only one componentsince the efficiency of the second component is exceedingly small at V,.However, when the tube is excited at a higher operating voltage, V,, theemission of the second component predominates since its efficiency is ata maximum and the first component luminesces with a reduced efiiciency.Operation at voltages between V and V will produce an emission colorgiven by the mixing of the colors of the two constituents according tothe well-known laws of color combination. For example, if the firstcomponent emits red light and the second green, operation of thepreviously described cathode ray tube at suitable voltages between V andV will produce orange, yellow, chartreuse, and various shades of thesecolors.

Another embodiment of the invention, which is an extension of the sameprinciples to more complex multi-component phosphors, has three or morecomponents, each component having a distinct efficiency characteristicand distinct emission color. Thus, a composite mixture of phosphorsconsisting of red, blue, and green-emitting components produces adisplay containing essentially all the colors attainable by theconventional aperture mask tube now in common use. Since the compositephosphor herein described can be applied to the face of the cathode raytube by the same techniques in common use in the manufacture ofmonochrome tubes, such as settling from a liquid suspension togetherwith some binding medium, the complexity and expense of manufacture oftubes made in accordance with the teachings of the instant invention aresubstantially reduced.

The novel features of this invention are set forth with particularity inthe appended claims. The invention itself, however, both as toorganization and method of operation, together with further objects andadvantages thereof, may be best understood by reference to the followingdescription taken in connection with the accompanying drawings, inwhich:

FIG. 1 illustrates schematically the cross section of a phosphorparticle made in accord with the teachings of the instant invention;

P10. 2 illustrates the efficiency with which the energy of an electronis converted to luminescence by phosphor particles made in accord withthe embodiment of FIG. 1;

FIG. 3 illustrates the efficiencies of a mixture of two kinds ofphosphors having different dimensions for various non-luminescent andluminescent regions;

FIG. 4 illustrates a portion of a luminescent screen of a color displaysystem in accord with an embodiment of the present invention.

FIG. 5 illustrates typical diffusion times and temperatures of aluminumin zinc-cadmium sulfide for producing phosphors with desiredaccelerating voltage characteristics for maximum efficiency.

FIG. 6 illustrates typical diffusion times and temperatures of cobalt inzinc-cadmium sulfide for producing phosphors with various dead voltages.

By way of example, the present invention is described with reference toFIG. 1 wherein there is illustrated a phosphor particle 10 having abasic structure comprising a substantially non-luminescent core 11, aluminescent region 12 and a second non-luminescent region 13 adjacentthe surface of the particle. The relative sizes of the three regions areonly shown schematically and it should be understood that the meanradius of region 11 may assume any value between zero and the radius ofthe entire particle. The inner and outer radii of the luminescent regionmay similarly fall in the entire range of possible radii.

The various regions of the particle may be composed of any reasonablytransparent solid state material, but of particular interest in theinstant invention are those materials which demonstrate comparativelyhigh luminous efficiency when excited by energized electrons and whichemit light of the desired color. Examples of these materials are the'alloys of zinc sulfide and cadmium sulfide, Zn,Cd ,,S, where x liesbetween zero and l, activated with copper, silver, or gold andcoactivated with halogens, chlorine, bromine, and iodine or aluminum orany other common coactivator. These phosphors are capable of emittingblue, cyan, green, yellow, orange, or red light. In the non-luminescentcore 11, either the activator or coactivator, or both, are absent whilethe non-luminescent outer layer 13 can be produced by adding one of thewellknown iron group killers such as iron, cobalt, or nickel.

Examples of red-emitting phosphors with greater luminous efficiency thanthe sulfides are the europium activated oxygen containing host materialssuch as yttrium vanadate, yttrium oxide, yttrium oxysulfide, andgadolinium oxide. As before, the relatively non-luminescent core region11, consists of the host material, such as yttrium oxide, containing noactivator, and the luminescent region 12, consists of yttrium oxidecontaining europium which has been grown, for example, from solution onthe outside of the non-luminescent core.

The efficiency as a function of electron accelerating potential forphosphors having the basic structure of FIG. 1 is shown by Curve A ofFIG. 2. The efiiciency is defined as the luminous emission intensitydivided by the electron beam power. When electrons are accelerated by apotential less than a threshold potential, V they do not penetrate thenon-luminescent outer layer 13 so the efficiency is exceedingly small.At higher potentials, some of the electrons reach the luminescent regionand the efficiency rises. At still higher potentials some of theelectrons penetrate through the luminescent region into thenon-luminescent core resulting in loss of efficiency. Consequently, theefficiency passes through a maximum at the potential V,, and thendecreases. The optimum accelerating potential for using the phosphor istherefore V The efficiency curves of two phosphors having differentdimensions for the luminescent and non-luminescent regions are shown inFIG. 3. The lower operating potential, V,, has been chosen to be nearthe potential which yields the maximum of Curve A and also less than thethresholdvalue of Curve B. The upper operating potential, V is near themax imum of Curve B while the value of Curve A is well below its maximumvalue. Consequently, operation of a cathode ray tube utilizing acomposite two-component-phosphor having efficiency characteristics suchas those illustrated in FIG. 3, produces essentially only the emissionof one of the two phosphors when operated at a potential V Similarly,operation at the potential V will yield predominantly the emission ofthe component having the higher excitation threshold.

As illustrated in FIG. 4, the instant invention may be embodied in athree-component color system comprising red, blue, and green-emittingphosphors, 14, 15 and 16, respectively. The sizes or thicknesses of thevarious regions of the different phosphors, the relative concentrationsof the different phosphors, and the operating potentials are chosen toprovide a color display with desired brightness and color rendition.Obviously the composite phosphor layer may have an evaporated film ofmetal, such as aluminum, added to the side closest the electron gun toserve as an electrical conductor and an optical reflector.

Typical phosphors useful in practicing the instant invention arecompositions of zinc-cadmium sulfide, Zn ,Cd S, (where x may vary fromzero to 1) with metal activators and halogen coactivators. To producephosphors by the method of the instant invention, a desired portion ofluminescent grade zinc-cadmium sulfide is heated in a flowing dryhydrogen sulfide stream for a sufficient period of time to drive outtraces of chlorides which would otherwise provide undesirableluminescence in the final product. This may be done, for example, byheating the zinc-cadmium sulfide for one hour at temperatures of 400,600, 800, and l,000 C successively. The zine-cadmium sulfide thusprepared, is mixed with antimony trioxide and heated in a vacuum or aninert atmosphere to promote crystal growth of 2 to 20 microns diametersize. This may be achieved by using the antimony trioxide in proportionsof 0.1 to 5 percent and heating the mixture to promote the growth ofcrystallites. Typical times and temperatures for achieving crystalgrowth range between 900 to 1,200 C for l to hours. The foregoingprocessing produces a non-luminescent well crystallized zinc sulfide.

To produce a luminescent region in the non-luminescent crystals, it isnecessary to activate the crystals with silver. This may be achieved,for example, by adding a soluble silver salt to a water slurry ofzinc-cadmium sulfide crystals. Specifically, silver nitrate may be usedto give a concentration of silver of 10" to 10 gram-atoms of aluminumper mole of zinc-cadmiurn sulfide. The slurry is then dried and mixedand the coactivator diffused into the zinc cadmium sulfide. Wherealuminum is used as the coactivator, diffusion may be achieved byheating in a hydrogen sulfide atmosphere for 5 minutes to 12 hours at400 to l,200 C.

At this point in the process, the phosphor has a non-luminescent coreand a luminescent outer region. To produce the phosphor as illustratedin FIG. 1, the following additional steps in the process are required.To .a water slurry of zinccadmium sulfide containing an activator and acoactivator, a killer impurity such as iron, cobalt, or nickel is added.For example, a soluble cobalt salt such as cobalt sulfate could be usedas a killer impurity. The zinc-cadmium sulfide powder is then washed anddried and subsequently heated in an inert gas atmosphere such as in anargon atmosphere for 5 minutes to 12 hours at 500 to l,200 C. Theresultant phosphor has an inert core and a non-luminescent outer regionseparated by a luminescent region. By varying the times and temperaturesset forth above, the parameters of the resultant phosphor can be alteredin accord with the requirements of the particular application.

While the foregoing description pertains to zinc-cadmium sulfidephosphors, it is to be understood that other phosphors could likewise beused. For example, to produce a redemitting phosphor consisting ofeuropium activated yttrium oxide in accord with the instant invention,yttrium nitrate and europium nitrate can be combined with an alkalihydroxide to promote crystal growth of a luminescent layer on top of anonluminescent core. This may be achieved, for example, as follows: To asuspension of crystallized yttrium hydroxide in 4-10 molar sodiumhydroxide, a solution of yttrium nitrate and europium nitrate is addedsuch that the concentration of europium is l to atom percent. Thesolution is stirred at a temperature from 20 to 100C from 1 to 16 hoursto promote growth of yttrium-europium hydroxide on the yttrium hydroxidecrystals. The resultant crystallites are then filtered and washed toremove any sodium hydroxide therefrom and subsequently are ignited inair to convert the yttrium hydroxide with an outer layer ofyttrium-europium oxide. This may be achieved, for example, by ignitingfor l to 12 hours at l,000 to l,200 C. The resultant phosphor will havean inert core and an adjacent luminescent surface region. Such aphosphor will luminesce at extremely low voltages and will continue toluminesce with increased efficiency until the electrons penetrate intothe non-luminescent core, at which time the efficiency will begin todecrease.

The method for making phosphor particles in accordance with theteachings of the instant invention can best be illustrated by referenceto the following specific examples.

EXAMPLE 1 diffusion, as described below, without the formation of theluminescent material. The zinc sulfide thus prepared is mixed with 1percent by weight of antimony trioxide and placed in a clear fusedquartz bulb which is evacuated and sealed off. The mixture is heated atl,100 C for 2 hours so as to promote crystal growth. The resultantparticles of zinc sulfide are small crystals 2 to 20 microns in size andare non-luminescent. To a water slurry of this crystallized zinc sulfideis added a dilute solution of silver nitrate so as to give a silverconcentration of 10" gram-atoms of silver per mole of zinc sulfide. Theslurry is dried at C and the resultant dry powder is mixed by tumbling.It is then heated in a flowing, dry hydrogen sulfide stream for twohours at 1,000 C in order to diffuse the silver completely into the zincsulfide particles and to remove antimony compounds by sublimation. Theresultant zinc sulfide with the diffused silver is essentiallynon-luminescent. To a water slurry of this zinc sulfide containingsilver is added a dilute solution of an aluminum salt such as thesulfate or nitrate so as to give an aluminum concentration of 10gramatoms of aluminum per mole of zinc sulfide. The slurry is dried andthe powder is mixed by tumbling. This dry powder is again heated in aflowing dry hydrogen sulfide stream for a period of time and temperatureselected so as to diffuse the aluminum into the zinc sulfide particlesto a desired distance. For example, 10 grams of zinc sulfide powder withdiffused silver and aluminum are heated at 900 C for 10 minutes. Thealuminum acts as a coactivator to produce a bright blue luminescencewhen present in the same region of a zinc sulfide particle with thesilver activator. At this point, the particles comprise a nonluminescentcore with an outer luminescent layer.

In order to produce the layered structure of the zinc sulfide particlesas illustrated in FIG. 1, the following additional deadening or killingis required. To a rapidly stirred suspension of 10 grams of the zincsulfide particles containing the diffused silver and partly diffusedaluminum in 50 milliliters of water, is added 0.15 milliliter of 0.1molar aqueous cobalt sulfate solution and 0.15 milliliters of 0.5 molarof aqueous ammonium sulfide solution. The zinc sulfide powder is thenallowed to settle from the suspension and the supernatant liquid isdecanted. The zinc sulfide powder is then washed with acetone, dried,and mixed by tumbling. The dry powder is then heated in a flowing dryargon atmosphere for 10 minutes at 700 C. The resultant phosphor willhave the structure illustrated in FIG. 1.

The phosphor particles thus produced may be settled in a thin layer on asuitable substrate in a manner well known in the art of making cathoderay screens. Under cathode ray excitation conditions, the phosphorscreen will have a characteristic luminescent efficiency as a functionof electron accelerating potential as illustrated in Curve A of FIG. 2with the maximum efficiency occurring at approximately 11 kilovoltsaccelerating potential and a dead voltage occurring at 4 kilovoltsaccelerating potential. The dead voltage, V,,, is obtained byextrapolating the straight line region of the efficiency vs. voltagecurve to the abscissa as shown in FIG. 2.

EXAMPLE 2 A modification of the procedure outlined in Example 1,produces phosphor particles having the structure illustrated in FIG. 1,but with the thickness of the outer non-luminescent layer 13, reduced tozero. This procedure is similar to that outlined in Example 1 exceptthat the portion of the foregoing deadening procedure involving theaddition of cobalt sulfate and ammonium sulfide and the subsequentheating in an argon stream is omitted. The phosphor particles producedby omitting the foregoing steps, exhibit a luminous efficiency undercathode ray excitation as a function of electron energy with a maximumoccurring at approximately 8 kilovolts and a dead voltage of 1 kilovoltas illustrated by Curve A of FIG. 2.

Whereas Example 2 produced phosphors having a maximum luminousefficiency occurring at approximately 8 kilovolts, by varying the timeand temperature of the heat treatment in the hydrogen sulfide stream ofthe zinc sulfide containing the diffused silver and aluminum, themaximum luminous efficiency may be made to occur at different electronenergy levels. For example, if the heating temperature and time werechanged from 900 C for 10 minutes to 900 C for 2 hours, the luminousefficiency will have a maximum at 12 kilovolts. As illustrated in FIG.5, longer heating times produce phosphors having maximum luminousefficiencies at higher electron energy levels. Additionally, from FIG.5, it can be seen that the heating may be performed either at lowtemperatures for long periods of time orat high temperatures for shortperiods of time. Similar results are obtained in either event; however,the longer heating times at the lower temperatures are advantageous whenlarge samples are to be heated simultaneously since effects due to thenon-uniform heating of the charge are minimized.

A further modification of the procedure outlined in Example I willproduce particles of phosphor having a structure as illustrated in FIG.1, but with the thickness of the inner non-luminescent core 11, reducedto zero. In the Example 3 below, a commercial Zn Cd ,S phosphoractivated with silver and coactivated with aluminum or the halogensbromine, or iodine and of such a composition so as to luminesce undercathode ray excitation with any desired color between blue and deep redis used as a starting material. The preparation of such phosphors iswell known in the art.

EXAMPLE 3 100 grams of a commercial zinc-cadmium sulfide phosphor isdeadened" as described in the latter part of Example 1, withproportionately increased amounts of cobalt sulfate and ammoniumsulfide. Portions of the dried and mixed phosphor are heated in 20 gramlots in a flowing argon stream at a temperature and for a period of timeselected so as to produce a phosphor having the desired dead voltage asdetennined by the thickness of the outer non-luminescent layer 13. Sucha phosphor will have a luminous efficiency under cathode ray excitationas a function of the electron energy as shown by Curve B of FIG. 2.

The dead voltage, which is related to the thickness of layer 13, can beused as one indication of the utility of the phosphor. By varying theheating time and temperature of the phosphor, the value of the deadvoltage can be controlled. The relationship between the dead voltage,heating temperature, and the heating time is illustrated in FIG. 6. Asillustrated therein, a decrease in temperature of 75 C is equivalent toan increase in heating time from 10 minutes to 2 hours.

EXAMPLE 4 A procedure for producing a red-emitting phosphor having thestructure illustrated in FIG. 1, but with the thickness of the outernon-luminescent layer reduced to zero is as follows: 10 milliliters ofan aqueous solution of yttrium nitrate containing 0.079 gram of yttriumper milliliter is added to 80 milliliters of an aqueous l molar sodiumhydroxide solution. The suspended precipitate of yttrium hydroxide isstirred rapidly and heated at 100 C for one hour. This procedureproduces needle-like crystals of yttrium hydroxide 2 to 6 microns inlength and about 2 microns in diameter. The particles of yttriumhydroxide first formed upon the addition of the yttrium solution to thesodium hydroxide solution are extremely small with a largesurface-to-volume ratio and have a higher solubility in the sodiumhydroxide solution than the larger crystals of yttrium hydroxide.Recrystallization will therefore occur so as to reduce thesurface-to-volume ratio in accord with the process frequently referredto in the technical literature as the aging, recrystallization, orripening of a fine precipitate. After 1 hour of heating at 100 C andagitation, l0 milliliters of a yttrium nitrate solution containing 0.079gram of yttrium per milliliter is mixed with l milliliter of an aqueouseuropium nitrate solution, containing 0.054 gram europium permilliliter; 4 milliliters of the mixed solution of yttrium nitrate andeuropium nitrate are added to the suspension of the crystallized yttriumhydroxide in the 10 molar sodium hydroxide. The suspension is againstirred rapidly for 16 hours without heating. The fine precipitate ofthe mixed europium-yttrium hydroxide will recrystallize and growprimarily on the surface of the already formed crystallites of theyttrium hydroxide by the same process described above. The precipitateis then separated from the sodium hydroxide solution by filtration andthoroughly washed until free of the sodium hydroxide. It is finallyignited in air for 2 hours at 1,050 C. This heating converts thehydroxides into corresponding oxides without change in the externalshape of the individual particles. The resultant phosphor, yttrium oxideactivated by europium, is excited by cathode rays to emit red light asis well known in the art. However, because of the structure resultingfrom the method of preparation described, the luminous efficiency undercathode ray excitation as a function of the electron energy will havecharacteristics as illustrated in Curve A of FIG. 2 with a maximum inthe efficiency occurring at approximately 6 kilovolts, and a deadvoltage of less than 1 kilovolt.

In the examples l-3, procedures were illustrated for preparing silveractivated zinc-cadmium sulfide phosphors having structures asillustrated in FIG. 1 with the thickness of the regions 11 and 13ranging from zero to that of the individual particle itself. Althoughspecific examples are given, it is obvious from the methods describedthat controlled changes in the time and temperature of the aluminum andcobalt diffusion makes it possible to prepare phosphor particles withany desired thickness for the regions 11 and 13. In Example 4, a rocessis described for preparing europium activated yttrium oxide phosphorparticles having the basic structure as illustrated in FIG. 1 with thethickness of region 13 equal to zero. By varying the weight of thesecond precipitate relative to the first precipitate, the thickness ofregion 12 can be controlled. By varying the thicknesses of the threeregions, the accelerating potential at which the maximum in luminescenceefficiency occurs and the dead voltage can be controlled. It is thuspossible to select parameters for the phosphors which are best suitedfor a particular color display application and prepare the phosphorsaccordingly.

EXAMPLE 5 grams of a green-emitting silver activated zinc-cadmiumsulfide phosphor is heated as described in Example 3. The heatingtemperature and time for the cobalt diffusion are 760 C for 10 minutes.This produces a green-emitting phosphor whose dead voltage under cathoderay excitation is approximately 6 kilovolts as illustrated in Curve B ofFIG. 2. The phosphor thus prepared is then silicized according to thewell-known art used in preparing phosphors for settling on cathode raytubes. This silicizing process assures adequate dispersion of thephosphor particles during settling of the phosphor on the substrate. Onesuch process consists of stirring the phosphor in a solution ofpotassium silicate and magnesium nitrate. The solutions of potassiumsilicate and magnesium nitrate are prepared, for example, in thefollowing proportions. For each 100 grams of phosphor to be coated, l.lmilliliters of DuPont K-ZOO potassium silicate solution and 20milliliters of a 3.8 percent by weight of magnesium nitrate solution areadded to 200 milliliters of water. The phosphor to be treated is addedto the solution and stirred for 30 minutes. The suspension is allowed tosettle and the liquid poured off. The phosphor is then washed indistilled water and sieved, for example, through a 200 mesh screen. 100grams of this silicized green phosphor are then admixed with 25 grams ofa red-emitting phosphor such as the europium activated yttrium vanadate.A portion of the mixed phosphors is then settled on a suitable substrateaccording to one of the well-known methods for making monochrometelevision screens by settling. The resultant screen will luminesce redunder cathode ray excitation when excited by approximately 4 kilovoltelectrons and will luminesce green when excited by approximately 14kilovolt electrons. At 5 kilovolts, the color is orange, at 6 kilovoltsthe color is yellow, at 7 kilovolts the color yellowish-white, at 8 and9 kilovolts the color is yellowishgreen. On the OLE. color mixturediagram, the x and y coordinates of this composite phosphor at thevarious accelerating voltages are shown in the following table:

Accelerating Voltage (in kilovolts) x y Since the color coordinates ofthe red phosphor by itself are x 0.650, y 0.350, and that the greenphosphor by itself has coordinates x 0.310 and y 0.605, it can be seenfrom the above table that a 4 kilovolts and 14 kilovolts, respectively,the colors of the mixed phosphors are close to the individual phosphorcomponents. This illustrates that essentially no desaturation of thecomponent colors has occurred.

Whereas the cathode ray tube described above is a twocomponent screenand the accessible colors are limited to a straight line connecting thecolor coordinates of the component phosphors in the OLE. diagram, athree-color component screen would make accessible colors within a colortriangle connecting the color coordinates of each of the three componentphosphors. Such a three-color component screen can be constructed byusing the above-mentioned greenemitting phosphor having anon-luminescent region 13 and a luminescent region 12 as shown in FIG.1, a blue-emitting phosphor having two non-luminescent regions 1 1 and13 and a luminescent region 12, also shown in FIG. 1, and a redemittingphosphor such as europium activated yttrium vanadate. Depending on thevoltage position of the maximum in the efficiency vs. voltage curve ofthe blue-emitting phosphor, the dead voltage values of the green andblueemitting phosphors and the relative efficiencies of the threephosphors, a suitable mixture is made so that the colors of the emittedlight under three different accelerating voltages are close to green,red, and blue.

Improved saturation of the colors at the high voltage levels can beobtained by using a red-emitting phosphor such as europium activatedyttrium oxide prepared as described above in Example 4. Thethree-component cathode ray tube screen is operated at three fixedaccelerating voltages and all the accessible colors within the colormixture triangle of the CH3. diagram are obtained by modulating theelectron beam current at each of the three voltages.

While only certain preferred embodiments have been shown by way ofillustration, many modifications and changes will occur to those skilledin the art.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electron energy sensitive phosphor consisting of at least aphosphor core region which is substantially non-luminescent andsurrounded by a luminescent region wherein said phosphor ischaracterized by an efficiency of emission which decreases above apredetermined electron energy.

2. An electron energy sensitive phosphor as recited in claim 1 whereinsaid phosphor particle has a substantially non-luminescent outer regionsurrounding said luminescent region and wherein said phosphor ischaracterized by an efficiency of emission which is a maximum at a firstpredetermined electron energy and which is substantially zero below asecond lower predetermined electron energy.

3. An electron energy sensitive phosphor as recited in claim 1 whereinsaid phosphor comprises a composition of zinc cadmium sulfide, Zn Cd ,S,where x varies from zero to 1.

4. An electron energy sensitive phosphor as recited in claim 3 whereinsaid luminescent region contains a selectively activated region of thematerial selected.

5. An electron energy sensitive phosphor as recited in claim 4 whereinan activator is selected from the group consisting of salts of silver,copper and gold.

2. An electron energy sensitive phosphor as recited in claim 1 whereinsaid phosphor particle has a substantially non-luminescent outer regionsurrounding said luminescent region and wherein said phosphor ischaracterized by an efficienCy of emission which is a maximum at a firstpredetermined electron energy and which is substantially zero below asecond lower predetermined electron energy.
 3. An electron energysensitive phosphor as recited in claim 1 wherein said phosphor comprisesa composition of zinc cadmium sulfide, ZnxCd(1 x)S, where x varies fromzero to
 1. 4. An electron energy sensitive phosphor as recited in claim3 wherein said luminescent region contains a selectively activatedregion of the material selected.
 5. An electron energy sensitivephosphor as recited in claim 4 wherein an activator is selected from thegroup consisting of salts of silver, copper and gold.